In my experience, a minimum of 6 fish in a school is often recommended when stocking freshwater aquaria. What is the specific reasoning behind that number 6? What is so special about it? Why not 5 or 7? I myself will recommend a minimum of 6 (but more is better if space allows) but am often at a loss to explain why 6 is the magic number. Or does the TFH Magazine have a better school size recommendation or method of determining how to stock schooling fish? If so, what is the specific reasoning behind it?

I understand why schooling fish need to be in schools, aggression dispersion being one of the reasons, but my question is about the specific minimum number. How was it derived? Why 6? Why not 5 or 7 or 10?

I don't know if I have an answer, but it is an excellent question. I find this sort of thing fascinating: why does something become the norm? Why is six the almost universally excepted minimum for a group of schooling fish? Here are some possibilities that come to mind...

1. Attrition. To safeguard against the possibility of fishkeepers losing some of the fish they just purchased so that they still have enough to protect against aggression, etc. (i.e. - five being enough, but get six just in case).

2. Pairing. Each fish has a buddy or a possible mate. Or you increase the likelihood of getting a breading pair if you have at least six. Also if one pair spawn the other four fish can keep each other occupied.

3. Psychology. Schooling fish obviously feel more at ease in a large school. Not all potential fishkeepers are going to buy a large school of fish. Many may be ignorant of the psychological and sociological needs of a fish species so by recommending six there are at least enough fish in the school for them all to feel comfortable. And keep in mind that six is really "at least six" or "a minimum of six".

4. Sales. Here's the cynic in me. It may be that the notion of six has been perpetuated by pet store and other sellers of tropical fish in order to increase sales. Over time it became an accepted norm as people began to see the benefits of stocking at least six of any schooling species. I admit that this isn't the most likely of theories, but it came to mind so I figured I'd include it.

5. Authority. Another possibility is that a long while back a respected authority such as Dr, Herbert Axelrod or someone of his caliber made the recommendation of six and the rest of the world decided that if so-and-so authority says six then who am I to argue?

6. Arbitrary. Who knows? Maybe someone picked the number six out of thin air and it stuck. You repeat something enough times (and get others to repeat it) eventually people begin to accept it. Look at how many falsehoods and "old wive's tales" there are in this hobby that persist even to today.

7. Esthetics. I covered this in one of my recent videos on my Aquatasy channel on YouTube. Human brains are wired to perceive an odd number of objects (or subjects) as random and an even number of them as ordered. Six fish schooling together appear more structured are more visually impressive than five.

8. Because. We do not question such things! Shut up and get six schooling fish and no one gets hurt!

Like I said, I don't have an actual answer. This is simply speculation on my part. I'm curious to know the truth of this.

I am on of the "odd is better" school. I prefer, 5, 7, 9 so on and so forth. Allows one to be the alpha and others to develop pairs. But for all the reason stated above, not only that, fish require social groups, just as us humans. The more, the more secure they feel.

The human brain can usually see 5 of something, and count it without assigning a number to each fish. More than that is harder to count. You have to look, and say, " One... two... three..." so if there are a minimum of 6 than you don't bother counting, you just say there is a school of fish.

Another reason: Among fish, (and almost everything else) there will be one or more that just do not want to hang out with the crowd. If you start with less than 6 and one wants to go explore somewhere else your school becomes 5 or fewer which is too few to look like a school.

I also agree with the concepts above: Even in schooling fish each is an individual. You can have more aggressive and less aggressive fish. When the group is too small then the most aggressive fish will repeatedly attack the least aggressive fish. When the group gets larger the less aggressive fish can lose itself in the crowd, and the more aggressive fish spreads out its aggression among the other fish so no one fish gets pestered to death. You still should watch their behavior. If you have one that is a lot more aggressive than average for the species I would remove that one.

I find a larger school even better. 2 dozen+ Cardinal Tetras in a large enough tank can sure behave like a true school, coordinating their swimming so that the school seems to flow as a unit. Fewer than that just do not give that group impression.

Crazygar wrote:I am on of the "odd is better" school. I prefer, 5, 7, 9 so on and so forth. Allows one to be the alpha and others to develop pairs.

Me too. Especially if it is more of a shoaling species I will get an odd number of them, usually five. If it is a tightly schooling species like Rummy-Nose Tetras or Harlequin Rasboras I get at least six, preferably more.

Diana wrote:The human brain can usually see 5 of something, and count it without assigning a number to each fish.More than that is harder to count. You have to look, and say, " One... two... three..."so if there are a minimum of 6 than you don't bother counting, you just say there is a school of fish.

That is an excellent observation, Diana! You are 100% correct.

Diana wrote:Another reason: Among fish, (and almost everything else) there will be one or more that just do not want to hang out with the crowd. If you start with less than 6 and one wants to go explore somewhere else your school becomes 5 or fewer which is too few to look like a school.

True. I had not thought about that.

Diana wrote:I find a larger school even better. 2 dozen+ Cardinal Tetras in a large enough tank can sure behave like a true school, coordinating their swimming so that the school seems to flow as a unit. Fewer than that just do not give that group impression.

Indeed! There is nothing quite as impressive as the sight of a bona fide school of fish.